Redefining the SI base units
3 November 2011
(PhysOrg.com) -- Metrology is poised to undergo a towards the experimental determination of the
profound change that will benefit scientists,
fundamental constants h, e, k and NA.”
engineers, industry and commerce – but which
almost no one will notice in daily life.
“Continual improvement in the means for making
precision measurements, namely the SI system, is
The international General Conference on Weights the core of NIST’s core mission,” says Patrick
Gallagher, Under Secretary of Commerce for
and Measures (CGPM) has approved a plan to
Standards and Technology and NIST Director.
redefine four of the seven base units of the
International System of Units (SI) in terms of fixed “PML’s fundamental metrology research helps
ensure that the SI system changes with the times.
values of natural constants. The initiative would
make possible new worldwide levels of consistency This helps NIST anticipate U.S. industry’s needs
and ultimately helps create jobs by ensuring the
and accuracy, simplify and normalize the unit
nation can make the highest quality, most indefinitions, and liberate the system from
dependence on the prototype kilogram, an artifact demand products.”
adopted in 1889 and still used as the world’s
The central philosophy of the impending redefinition
physical standard for mass.
is that instead of defining an SI unit per se, the
On Oct. 21, 2011, CGPM, the diplomatic body that CGPM will specify exact values value for a set of
fundamental constants which will set the scale for
has the authority under the Meter Convention to
the SI units. The values of those physical constants
adopt such a sweeping change, approved a
resolution declaring that the kilogram, the ampere, will reflect the most accurate determinations
the kelvin and the mole, “will be redefined in terms available from NIST, NMIs, and academic
institutions at the time of implementation.
of invariants of nature; the new definitions will be
based on fixed numerical values of the Planck
Three decades ago, the same schema was used
constant (h), the elementary charge (e), the
for the first time to redefine the meter: In 1983,
Boltzmann constant (k), and the Avogadro
CGPM defined the meter by setting an exact fixed
constant (NA), respectively.”
value of the speed of light in vacuum (299 792 458
m s-1), citing “the excellent agreement among the
That action follows – and results directly from –
decades of pioneering metrology research around results of wavelength measurements on the
radiations of lasers . . . .”
the globe, some of it accomplished by various
groups at NIST and its antecedent, the National
“That is kind of a model for the way they’re
Bureau of Standards (NBS), that are now part of
PML. And it echoes the recommendations made by planning to redefine the other units now,” says
three PML scientists and two European colleagues Peter Mohr of PML’s Quantum Measurement
Division, one of the authors of the 2006 Metrologia
in an influential 2006 paper in Metrologia.
paper.
The change will not be implemented until the
The meter redefinition relied heavily on a then-new
technical requirements for agreement and
method, devised by NBS researchers in Boulder, to
uncertainties are met. (The next scheduled
measure the speed of light, and since then PML
meeting of the CGPM is in 2014.) In the interim,
has been substantially involved in all aspects of the
more work will be required: CGPM has called for
“New SI.” In particular, PML groups have made
further reductions in measurement uncertainty
major, sustained contributions to the impending
before the “New SI” can be implemented, and
redefinitions of the kilogram and the kelvin, the
encouraged national metrology institutes (NMIs)
implementation of which will take place under the
and other institutions to “maintain their efforts
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auspices of the International Bureau of Weights and transferred last year to Canada’s National
Measures (BIPM), an intergovernmental
Research Council (NRC).
organization under the authority of the CGPM.
NIST PML has operated a vacuum-enclosed watt
balance for years. In 2007, the PML team reported
The Kilogram
a relative uncertainty of 3.6 x 10-8. Other watt
The kilogram – the only SI unit whose current
balance devices are either operating or under
definition is based on a manufactured object – is
development in Canada, Switzerland, France,
literally embodied in the original platinum-iridium
China, and at the BIPM.
prototype, the mass of which appears to have
drifted over time in the range of a few parts in 108 The other approach to determining h, pursued by a
in the course of the past 100 years. Much of that
single large international collaboration, involves
evidence comes from a device designed and made “counting” the number of atoms (via unit cell
by a PML predecessor: Beginning in 1970, the
volume of a crystal) in each of two highly pure 1 kg
BIPM conducted its mass comparisons using a
single-crystal enriched silicon spheres about 94
balance called NBS-2.
mm in diameter. The result provides a
determination of the Avogadro constant, which in
In the new system, the kg will be defined by fixing turn can be used to obtain h using the well-known
the value of the Planck constant, h, at 6.626 06X x values of other constants.
10-34 joule second. (The final “X” represents one or
more digits to be determined by results of future
PML is not actively involved in Avogadro
experiments.) That definition, succinctly proposed experiments at the present time, although some of
by Mohr and fellow PML scientist Barry Taylor in
the best early work on Si crystals was done about
1999, links h to mass based on two fundamental
40 years ago by NBS researchers led by Richard
2
relationships: E = hv and E= mc .
Deslattes. Measurements of crystals “go back a
really long time,” Taylor says, “but Deslattes was
“It didn’t just come out of the blue. It was
the first person to plan out a truly full-scale attack
germinating for quite a while because the idea of
on the Avogadro constant using silicon spheres, a
using a watt balance to monitor the kilogram had
combined X-ray and optical interferometer, and
been around for a while,” says Taylor. “Peter and I research on isotopic abundances,” among other
were returning from a talk on the watt balance in
things.
1999 when the idea of using it to determine mass
from the Planck constant rather than vice versa
Taylor, Mohr and NIST watt-balance veteran
came up. That led to the paper.”
Richard Steiner believe that, eventually, the watt
balance is likely to become the standard device: “It
Some experts have set a target relative uncertainty seems that the watt balance is the more likely
of 2 x 10-8 (two parts in a hundred million) for the
candidate,” Mohr says, “because there are six
kilogram. “That’s a very high bar,” says Mohr. But independent experiments, whereas there’s only
two approaches that are physically very different
one silicon experiment, albeit with two spheres.”
are getting close to that threshold.
Steiner, who worked for years on the NIST watt
One employs a device called a watt balance, which balance and was the first author on the 2007 paper,
measures the force required to balance a 1 kg
says “The CGPM resolution leaves the options
mass artifact against the pull of Earth’s gravity by open for defining mass from the Planck constant or
monitoring the voltage and current (hence the
the Avogadro constant. Whether the watt balance
name “watt”) involved in doing so. The first such
or Si spheres are used as the mass realization
device, pioneered at the United Kingdom’s
system will come down to practicality. Presumably,
National Physical Laboratory (NPL) “was originally with the practical goal for the watt balances of
involved in the realization of the ampere,” Taylor
measuring masses other than a few expensive
says. A more recent version of the balance was
platinum-iridium cylinders, it should have a greater
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potential usefulness as opposed to measuring even “Both the NRC and NIST systems have a good 20
fewer and even more expensive silicon spheres.
years of people scratching their heads about what
kinds of systematic mistakes they could be making.
"Both research areas need to be continued, but
The NIST figures are quite consistent, and we’ve
they are not equally practical in the long term for
been beating the error bars down over time.
calibration dissemination. Even the watt balances Nonetheless, we’re doing a top-to-bottom analysis
have short-term (a few months) uncertainty limits
of the experiment, to see if something new crops
that have not been fully characterized. Most
up. If there are problems, it’s most likely that we
published values rely upon long-term data of years, will see a compounding of multiple small things.
where many uncertainty contributions are
And the same is true of the NRC balance.”
considered to average out.”
The next run on the PML balance is scheduled for
PML is actively undertaking two watt-balance
February 2012, with more to follow. But even if the
initiatives: A top-to-bottom investigation of the
analysis uncovers a number of ways to reduce
existing NIST device in search of any sources of
relative uncertainty, the chances of getting to the
error that can be eliminated to improve its
CGPM’s preferred figure of 2 x 10-8 are “slim,”
uncertainty figures, and construction of an entirely Pratt says. “There’s a limit to what’s achievable.
new watt balance in a dedicated, climate-controlled We might be able to coax some more out of it, but I
facility.
doubt it.” On the other hand, CGPM may decide
that its current target is too stringent. “Our hope,”
“The one that’s out there now you can think of as a Taylor says, “is that when push comes to shove,
thoroughbred,” says Jon Pratt, who heads the
three or four in 108 will be acceptable.”
Fundamental Electrical Measurements group in the
Quantum Measurements Division. “It’s been bred The Kelvin
for speed, it wants to get a value of h, and it wants
to do it once. The next one we’re building is much The SI base unit for thermodynamic temperature is
more of a Clydesdale. We want it to be much more the kelvin. It was first defined in modern form in
of a workhorse, able to measure masses on a
1954, and restated in 1967 as “the fraction
consistent basis.” It will employ a 1 metric ton
1/273.16 of the thermodynamic temperature of the
permanent magnet, in contrast to the
triple point of water.” That would seem to make it
superconducting magnet in the existing unit, and
fairly easy to realize. But in fact, that temperature
will take several years to complete.
depends critically on the relative proportions of
isotopes of hydrogen and oxygen in the water
Meanwhile, Pratt is heading the re-examination of (among other factors). So in 2005, the definition
the existing balance – a timely endeavor. “The
was amended to include a recipe:
NRC has a new number using the watt balance
they purchased from NPL that is very close to the “This definition refers to water having the isotopic
latest value from the international Avogadro
composition defined exactly by the following
project,” Pratt says. “That certainly doesn’t mean
amount of substance ratios: 0.000 155 76 mole of
2
that they’re both definitely right. But it’s a good
H per mole of 1H, 0.000 379 9 mole of 17O per
opportunity for us to take a new look at the NIST
mole of 16O, and 0.002 005 2 mole of 18O per mole
apparatus. There’s a difference of about 250 parts of 16O.” That formula, known somewhat confusingly
per billion between the NRC balance and ours that as Vienna Standard Mean Ocean Water even
we want to work with the NRC to understand. They though it contains no salt or other solutes or
appear to have made significant strides in
impurities, is not easy to reproduce in the
tightening up the uncertainty associated with
laboratory.
operating that particular balance, and if both our
estimated uncertainties are to be believed, then
But even with the right water, the definition will only
differences exist between our balances that are yet yield the temperature at the triple point – a
to be properly accounted for.
temperature at which most of the sophisticated
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“primary” thermometers employed by NMIs cannot
be used. Those devices rely on the more difficult
measurement of fundamental thermodynamic
quantities, and particularly the average energy in a
mechanical degree of freedom. The Boltzmann
constant links that energy to temperature.
In the “New SI,” the kelvin will be defined
specifically in terms of the Boltzmann constant, k
(sometimes written as kB), which is known to a very
high degree of accuracy – thanks in large measure
to PML scientist Mike Moldover and colleagues,
who set the standard for precision measurement
over 20 years ago using an acoustic method.
Bureau of Standards, and specified k to a relative
uncertainty of 1.8 x 10-6.
“His result has stood the test of time,” says Taylor.
“It was not until last year when somebody obtained
a value with an uncertainty they claim is smaller
than his. Mike’s work was a real tour de force.”
But it required overcoming host of problems. The
spherical acoustical chamber held about three liters
of gas, the isotopic composition of which had to be
known exactly. To determine the volume, the team
weighed the mercury required to fill it, using the
density of mercury measured by A.H. Cook in 1957
and 1961. The temperature had to be determined
to within 0.25 mK. “And we had to learn a lot of
little things that affect the results in small ways,”
Moldover says. “The way you measure a resonant
frequency is very tricky. Some heat flows from the
gas into the wall, so you have to account for that.
Also walls recoil a little bit, and that changes the
resonant frequency, so you have to know precisely
how elastic the metal is.”
“The faster the particles go, the faster sound
moves in the gas,” Moldover explains. “So in the
1980s, our group developed an approach to
measuring the speed of sound which was more
accurate than anyone had done before. We
measured the speed of sound in argon gas near
the temperature of the triple point of water. By
combining the speed of sound with the average
mass of the argon atoms, we deduced the average
kinetic energy of the argon atoms. The combination The effort continued past 1988, as Moldover
of the kinetic energy and the temperature of the
applied his method to the International
argon gave us the Boltzmann constant.”
Temperature Scale published in 1990 (known as
ITS-90), which lists key temperature points for
Moldover’s original motivation was to check a
various substances. “We measured the
British result. “The folks at one of our rival labs,
thermodynamic temperature for these different
NPL, determined k from an acoustic measurement points and got values for the difference between
of the gas constant in 1976,” he recalls. “Their
ITS-90 and the thermodynamic temperature that
number was surprisingly large – well off from what were about five times more accurate than anyone
people thought it was.
had gotten before. They have since been
reproduced elsewhere.”
“My boss said, ‘I’m sure this is wrong. You guys
think of a way to do a better job of measuring it.’
Today, PML remains intensely involved in a range
So I set about using acoustic resonators. As it
of experiments on k. “NIST is one of the NMIs that
happened, very soon after the NPL results were
have determined the Boltzmann constant using two
announced, two Oxford scientists pointed out that different methods – acoustic gas thermometry and
the NPL people had made an error in extrapolating Johnson noise thermometry,” says Greg Strouse,
to zero pressure. The NPL researchers corrected who leads PML’s Temperature and Humidity
their error, and their number came back to what
Group.
people had expected. But we kept going anyway,
because we thought we could do five times better. “But unique to NIST, PML’s Sensor Science
And we did.”
Division (SSD) intends to determine the Boltzmann
constant with a third method – spectral-radiance
It ended up taking 12 years. When the group’s
thermometry – to further our confidence in the
figures were published in 1988, the paper filled 60 assigned value and ensure that the impact of the repages in the Journal of Research of the National
definition on industry is small,” Strouse says. “The
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NIST dissemination of both the thermodynamic
temperature scale and ITS-90 expands the role of
NIST, enhances the link between contact and noncontact thermometry, and allows for improved
temperature measurement at both high and low
temperatures.
“While the dual dissemination of temperature will
not impact most industries, the dissemination of
thermodynamic temperature will improve the
assigning of values to material thermo-physical
properties. By advancing detector-based, Johnson
noise, and acoustic thermometer technology, PML
will improve the uncertainty in the realization and
dissemination of the thermodynamic temperature
scale. The use of thermodynamic thermometers will
find application in high-temperature environments
(e.g., metal refractory), hazardous environments
(e.g., nuclear), and space monitoring of Earth’s
climate change. Eventually, the use of the ITS-90
defining standards will be gradually replaced by
thermodynamic temperature defining standards.
NIST PML SSD is positioning itself for this change
by developing thermodynamic thermometers for
standards and industrial use.”
Provided by National Institute of Standards and
Technology
APA citation: Redefining the SI base units (2011, November 3) retrieved 17 June 2017 from
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